hw/s390x/ipl: Fix endianness problem with netboot_start_addr

The start address has to be stored in big endian byte order
in the iplb.ccw block for the guest.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1499268345-12552-1-git-send-email-thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>